1. Field of the Invention
The invention relates to a semiconductor device, and more particularly to a semiconductor device including an electrically erasable programmable read only memory having a two-gate structure of a floating gate and a control gate deposited on the floating gate.
2. Description of the Related Art
An electrically erasable programmable read only memory (hereinafter, referred to simply as xe2x80x9cEEPROMxe2x80x9d) generally includes, as a memory cell, MISFET memory transistor having a two-gate structure of a floating gate and a control gate formed on the floating gate. Data is written into or eliminated from the two-gate type EEPROM by introducing electric charges into or discharging electric charges from a floating gate.
For instance, data is written into the two-gate type EEPROM by introducing channel hot electrons, generated in drain regions, into a floating gate, whereas data is eliminated from EEPROM by introducing electrons into a source, for instance, by virtue of Fowler-Nordheim tunneling.
A conventional method of fabricating a two-gate type memory cell array is explained hereinbelow with reference to FIGS. 1, 2 and 3A to 3D, wherein FIG. 1 is a plan view of a conventional two-gate type memory cell array, FIG. 2 is a plan view illustrating the memory cell array being fabricated, and FIGS. 3A to 3D are cross-sectional views of the memory cell array taken along the line IIIxe2x80x94III in FIG. 1, showing respective steps of a method of fabricating the memory cell array.
As illustrated in FIG. 3A, a p-type well 2 is formed in a p-type semiconductor substrate 1 in a region where a memory cell array is to be formed. Then, a plurality of field insulating films 3 is formed in the form of islands by selective oxidation. The field insulating films are not illustrated in FIG. 3A, but are insulated in FIG. 2.
Then, a first gate insulating film 4 is formed all over the p-type well 2, and a first polysilicon layer 5a is formed all over the first gate insulating film 4 for forming a floating gate. Then, impurities such as phosphorus (P) are doped into the first polysilicon layer 5a by thermal diffusion or ion-implantation to thereby lower a resistance of the first polysilicon layer 5a. Then, as illustrated in FIG. 2, the first polysilicon layer 5a is patterned into a plurality of layers 5a in parallel with each other so that the layers 5a extend perpendicularly to word lines which will be formed later, in order to define a width thereof in a direction of a channel width of a floating gate.
Then, a second gate insulating film 6 is formed all over the product, and a second polysilicon layer 7a is formed over the second gate insulating film 6. Then, as illustrated in FIG. 3A, a patterned photoresist film 18a is formed on the second polysilicon layer 7a by photolithography and dry etching. The photoresist film 18a has a pattern for forming word lines.
Then, as illustrated in FIG. 3B, the second and first polysilicon layers 7a and 5a are patterned with the patterned photoresist film 18a being used as a mask, to thereby form control gates 7 and floating gates 5. After removal of the photoresist film 18a, impurities such as arsenic (As) are ion-implanted into the product with the deposited gates 5 and 7 and the field insulating films 3 being used as a mask, to thereby form drain regions 8a and source regions 8b. 
Then, as illustrated in FIG. 3C, sidewall spacers 9 are formed around a sidewall of the deposited gates 5 and 7 of each of memory cells in order to cause CMOS transistors located outside memory cell array regions to have a LDD-structure. Thereafter, a first interlayer insulating film 10 is deposited all over the product. The first interlayer insulating film 10 is composed of boron phospho silicate glass (BPSG), and has a thickness in the range of 6000 to 8000 angstroms.
Then, there is formed a photoresist film 18e having a hole above the drain region 8a. Then, the first interlayer insulating film 10 is etched with the photoresist film 18e being used as a mask, to thereby form a contact hole 11 leading to the drain region 8a. 
After removal of the photoresist film 18e, aluminum alloy is deposited by sputtering by a thickness in the range of 4000 to 6000 angstroms. Then, the aluminum alloy is patterned by photolithography and dry etching to thereby form bit lines 12 extending perpendicularly to the word lines. Then, the product is entirely covered with a passivation film 16 composed of PSG and having a thickness of about 5000 angstroms. Thus, there is completed a memory cell array.
While the above-mentioned method is being carried out a region 3a (a hatched region in FIG. 2) which is sandwiched between the field insulating films 3 and will become a source region is exposed to etching twice, namely, when the first polysilicon layer 5a is patterned and when the second polysilicon layer 7a is patterned. When the first polysilicon layer 5a is patterned, the region 3a is covered merely with the thin first gate insulating film 4 after the first polysilicon layer 5a has been etched. Hence, the first gate insulating film 4 is first removed, and then, the p-type semiconductor substrate 1 is undesirably etched. In addition, when the second polysilicon layer 7a is patterned, the region 3a is covered merely with the thin second gate insulating film 6 after the second polysilicon layer 7a has been etched. Hence, the p-type semiconductor substrate 1 is undesirably further etched.
As a result, as illustrated in FIG. 4 which is a cross-sectional view taken along the line IVxe2x80x94IV in FIG. 1, there is formed an undesirable recess 19 at a surface of the semiconductor substrate 1. The undesirable recess 19 causes junction leakage therein, which poses a problem that data-writing and data-eliminating properties are deteriorated.
If a diffusion layer had a depth shallower than a depth of the recess 19, there is formed a breakage in a source region at the recess 19, since impurities are not ion-implanted into an inner sidewall of the recess 19. This causes a reduction in a fabrication yield.
The above-mentioned problem can be solved by a semiconductor device structure as suggested in Japanese Unexamined Patent Publications Nos. 3-52267 and 3-126266, for instance. Hereinafter is explained the suggested structure with reference to FIGS. 5, 6, 7 and 8A to 8D, wherein FIG. 5 is a plan view of the suggested memory cell array, FIG. 6 is a cross-sectional view taken along the line VIxe2x80x94VI in FIG. 5, FIG. 7 is a cross-sectional view taken along the line VIIxe2x80x94VII in FIG. 5, and FIGS. 8A to 8D are cross-sectional views taken along the line VIxe2x80x94VI in FIG. 5, showing respective steps of a method of fabricating the suggested memory cell array.
The suggested memory cell array is characterized by that a plurality of the field insulating films 3 extend perpendicularly to the word lines 7, and that the common source line 17a connecting the source regions 8b to each other in a direction in which the word lines 7 extend is formed to extend perpendicularly to the field insulating films 3. Hereinafter is explained a method of fabricating the suggested memory cell array, with reference to FIGS. 8A to 8D.
As illustrated in FIG. 8A, a p-type well 2 is formed in a p-type semiconductor substrate 1 by introducing p-type impurities into the semiconductor substrate 1 and thermally diffusing the p-type impurities and the semiconductor substrate 1. Then, a plurality of field insulating films 3 are formed on a principal surface of the p-type well 2 by selective oxidation so that the field insulating films 3 extend in parallel with one another, but perpendicularly to word lines which will be formed later. The field insulating films 3 are not illustrated in FIG. 8A, but are illustrated in FIG. 5.
Then, a first gate insulating film 4 and then a first polysilicon layer 5a are formed all over the product. Then, impurities such as phosphorus (P) are ion-implanted into the first polysilicon layer 5a to thereby lower a resistance of the first polysilicon layer 5a. Then, as illustrated in FIG. 2, the first polysilicon layer 5a is patterned into a plurality of layers 5a in parallel with each other in order to define a width thereof in a direction of a channel width of a floating gate. When the first polysilicon layer 5a is patterned, the thick field insulating films 3 exist below a region where the first polysilicon layer 5a is etched, which ensures that the substrate 1 is not etched, and hence a recess such as the recess 19 illustrated in FIG. 4 is not formed.
Then, a second gate insulating film 6 is formed all over the product, and a second polysilicon layer 7a is formed over the second gate insulating film 6. Then, impurities such as phosphorus (P) are ion-implanted into the second polysilicon layer 7a to thereby lower a resistance thereof. Then, as illustrated in FIG. 8A, a patterned photoresist film 18a is formed on the second polysilicon layer 7a by photolithography and dry etching. The photoresist film 18a has a pattern for forming word lines.
Then, as illustrated in FIG. 8B, the second and first polysilicon layers 7a and 5a are patterned by etching with the patterned photoresist film 18a being used as a mask, to thereby form control gates 7 and floating gates 5. After removal of the photoresist film 18a, n-type impurities are ion-implanted into the product with the deposited gates 5 and 7 and the field insulating films 3 being used as a mask, to thereby form drain regions 8a and source regions 8b. 
Then, as illustrated in FIG. 8C, sidewall spacers 9 are formed around a sidewall of the deposited gates 5 and 7 of each of memory cells. Thereafter, a first interlayer insulating film 10 is deposited all over the product by chemical vapor deposition (CVD). The first interlayer insulating film 10 is composed of silicon dioxide. Then, the first interlayer insulating film 10 is etched in selected regions to thereby form contact holes C1 reaching a surface of the source regions 8b and contact holes C2 reaching a surface of the drain regions 8a. 
Then, as illustrated in FIG. 8D, an electrically conductive layer composed of polysilicon is formed all over the product, and then patterned to thereby form a common source line 17a and an extended bit line 17b. The common source line 17a connects the source regions 8b in a direction in which the word lines extend. The extended bit line 17b makes electrical contact with the drain region 8a through the contact hole C2, and covers a portion of the first interlayer insulating film 10 around the contact hole C2 therewith. The electrically conductive layer from which the common source line 17a and the extended bit line 17b are formed may be composed of refractory metal, silicide thereof, or polycide thereof, as well as polysilicon.
Then, a second interlayer insulating film 13 composed of BPSG is deposited all over the product. Thereafter, a photoresist film 18c is formed, and then, patterned by photolithography and dry etching so as to have an opening above the extended bit line 17b. Then, the second interlayer insulating film 13 is etched with the patterned photoresist film 18c being used as a mask, to thereby form through-holes 14 reaching the extended bit line 17b. 
After removal of the photoresist film 18c, aluminum alloy is deposited by sputtering. Then, the aluminum alloy is patterned by photolithography and dry etching to thereby form bit lines 12 (see FIGS. 5, 6 and 7) extending perpendicularly to the word lines. Then, the product is entirely covered with a passivation film 16 (see FIGS. 4, 6 and 7) composed of PSG. Thus, there is completed a non-volatile semiconductor memory device as illustrated in FIGS. 4 to 7.
In accordance with the above-mentioned method, when the first polysilicon layer 5a is etched, the thick field insulating films 3 exist below a region to be etched. When the second and first polysilicon layers 7a and 5a are patterned to thereby form the control gate 7 and the floating gate 5, a region where only a single polysilicon layer is etched is a region located above the field insulating regions 3. Hence, the above-mentioned undesirable recess 19 caused by etching a polysilicon layer is not formed. Accordingly, there is solved a problem that junction leakage occurs due to the recess, and resultingly data-writing and data-eliminating properties are deteriorated, and that a fabrication yield due to the breakage in a source region is reduced.
A semiconductor device including a high-rate CMOS logic circuit is generally designed to have two or more wiring layers. When a non-volatile memory is formed on a common semiconductor substrate on which a high-rate CMOS logic circuit is also formed, it is required that an increase in the number of additional fabrication steps is avoided and that the non-volatile memory is small in size, in order to reduce fabrication costs and integrate the device in a higher density.
In the conventional method having been explained with reference to FIGS. 5, 6, 7 and 8A to 8D, the common source line is formed of the electrically conductive layer composed of electrically conductive material such as polysilicon, after the contact hole has been formed. Hence, the above-mentioned conventional method has a problem that the number of additional fabrication steps is increased relative to the number of steps for fabricating CMOS logic circuit having two or more wiring layers, and hence, fabrication costs are also increased.
In addition, since the common source line is formed of an electrically conductive layer composed of polysilicon, the common source line unavoidably has high resistivity, which causes problems that data-writing and data-eliminating properties of a non-volatile memory are deteriorated, and that a speed at which a memory cell reads out data is reduced.
The common source line may be designed to have a smaller resistance by increasing an area of the electrically conductive layer and/or forming a backing wiring layer composed of aluminum. However, this makes it difficult to reduce a size of a memory cell, and reduce fabrication costs per a chip.
It is an object of the present invention to provide an electrically erasable programmable read only memory which is capable of being formed commonly on a semiconductor substrate on which a high-rate CMOS semiconductor device is also formed, without an increase in the number of additional fabrication steps, and also capable of writing data thereinto and reading data therefrom at a high rate without an increase in a cell size.
The above-mentioned object can be accomplished by presenting a non-volatile memory including memory cells having a floating gate and a control gate doubling as a word line, field insulating films each extending perpendicularly to word lines to thereby electrically insulate the memory cells from one another, a common source line extending in parallel to the word lines to thereby connect source regions of the memory cells to one another, and a bit line extending perpendicularly to the word lines to thereby connect drain regions of the memory cells to one another. The common source line may be formed of a first metal wiring layer, and the bit line may be formed of a second metal wiring layer.
Specifically, in one aspect of the present invention, there is provided an EEPROM semiconductor device including (a) a plurality of field insulating films each extending perpendicularly to word lines, (b) a plurality of memory cells arranged in a matrix, each memory cell including a floating gate, a control gate formed on the floating gate and doubling as a word line, and source and drain regions located at either sides of the control gate, (c) a common source line extending in parallel with the word lines and connecting source regions of the memory cells with each other, and (d) a first bit line extending perpendicularly to the word lines and connecting drain regions of the memory cells with each other.
The common source line may be constituted of a first metal wiring layer, which is preferably composed of aluminum. The bit line may be constituted of a second metal wiring layer, which is preferably composed of aluminum.
The EEPROM semiconductor device may further include a plurality of second bit lines formed above the drain regions of the memory cells, in which case, it is preferable that the first bit line connects the second bit lines with one another.
It is preferable that both the second bit lines and the common source line are constituted of a first metal wiring layer, which is preferably composed of aluminum.
The EEPROM semiconductor device may further include CMOS logic circuit including both the common source line and the first bit line, and formed on a common semiconductor substrate.
There is further provided an EEPROM semiconductor device including (a) a plurality of field insulating films each extending perpendicularly to word lines, (b) a plurality of memory cells arranged in a matrix, each memory cell including a floating gate, a control gate formed on the floating gate and doubling as a word line; and source and drain regions located at either sides of the control gate, (c) a first bit line extending perpendicularly to the word lines and connecting drain regions of the memory cells with each other, and (d) a first common source line extending in parallel with the word lines and connecting source regions of the memory cells with each other.
The EEPROM semiconductor device may further include a plurality of second common source lines formed above the source regions of the memory cells, in which case, the first common source line preferably connects the second common source lines with one another.
It is preferable that both the second common source lines and the bit line are constituted of a first metal wiring layer, which is preferably composed of aluminum.
The EEPROM semiconductor device may further include backing wiring layers each of which is connected to the word lines at every certain number of bits, in which case, it is preferable that both the backing wiring layers and the first common source lines are constituted of a second metal wiring layer.
In another aspect, there is provided a method of fabricating an EEPROM semiconductor device, including the steps of (a) forming a plurality of field insulating films in parallel on a semiconductor substrate, (b) forming a first gate insulating film in each of active regions, (c) forming a plurality of first polysilicon layers in parallel with one another perpendicularly to word lines, (d) forming a second gate insulating film and a second polysilicon layer all over the product resulting from the step (c), (e) patterning the second polysilicon layer, the second gate insulating film, and the first polysilicon layer to thereby form a control gate and a floating gate, (f) forming drain and source regions, (g) forming a first interlayer insulating layer all over the product resulting from the step (f), (h) forming a first metal wiring layer which is patterned so as to form both a common source line extending in parallel with the word lines and connecting source regions to one another, and an extended bit line connecting the drain region to a bit line, (i) forming a second interlayer insulating layer all over the product resulting from the step (h), and (j) forming a second metal wiring layer which is patterned so as to form a bit line connecting the drain regions to one another.
The second gate insulating film may have a three-layered structure of oxide/nitride/oxide films.
There is further provided a method of fabricating an EEPROM semiconductor device, including the steps of (a) forming a plurality of field insulating films in parallel on a semiconductor substrate, (b) forming a first gate insulating film in each of active regions, (c) forming a plurality of first polysilicon layers in parallel with one another perpendicularly to word lines, (d) forming a second gate insulating film and a second polysilicon layer all over the product resulting from the step (c), (e) patterning the second polysilicon layer, the second gate insulating film, and the first polysilicon layer to thereby form a control gate and a floating gate, (f) forming drain and source regions, (g) forming a first interlayer insulating layer all over the product resulting from the step (f), (h) forming a first metal wiring layer which is patterned so as to form both a bit line extending almost in parallel with the field insulating films and connecting drain regions to one another, and an extended common source line connecting the source region to a later mentioned common source line, (i) forming a second interlayer insulating layer all over the product resulting from the step (h), and (j) forming a second metal wiring layer which is patterned so as to form a common source line connecting the source regions to one another.
The method may further include the step of forming backing wiring layers connecting to the control gate at a certain interval, in which case, the backing wiring layers are preferably constituted of the second metal wiring layer.
The above and other objects and advantageous features of the present invention will be made apparent from the following description made with reference to the accompanying drawings, in which like reference characters designate the same or similar parts throughout the drawings.